Peat Stratigraphy

SUB-ATLANTIC (cool and wet)

"C DATES

AAAA 850 AAM 1680

SUB-BOREAL (warm and dry)

ATLANTIC

BOREAL

PRE-BOREAL

YOUNGER DRYAS

ALLER0D

OLDER DRYAS

S0LUNG

OLDEST DRYAS

4240

4240

6050

6910

6910

FIGURE 2.2 "Climatic discontinuities" revealed by analysis of over 800 l4C dates on stratigraphic discontinuities in paleoenvironmental (primarily botanical) records (based on data inWendland and Bryson, 1974). Major and minor discontinuities are shown by large and small jagged lines, respectively. Time limits of the Hyp-sithermal and the Altithermal are from Deevey and Flint (1957).The Blytt-Sernander scheme of Scandinavian peat stratigraphy was developed before l4C dating techniques were available. It is based on changes in peat growth that were considered to be climate related. Radiocarbon dates now indicate that the boundaries are not precise, but vary over the ranges indicated (based on summaries by Godwin, 1956 and Deevey and Flint, 1957). Objective analyses of peat stratigraphy indicate that the "classic" stages of peat stratigraphy may not be of regional significance after all (except for the Sub-Boreal/Sub-Atlantic transition at about 2500 years B.R) (Birks and Birks, 1981). Nevertheless, the descriptors (Atlantic, Sub-Atlantic, etc.) are still commonly used to refer to a particular time period, albeit vaguely defined, both climatically and chronologically. Late Glacial/Early Holocene chrono-zones are from Mangerud et al. (1974).

synchronous climatic changes, presumably brought about by some large-scale forcing. In particular the period 2760-2510 yr B.P. (the beginning of sub-Atlantic time) stands out in both palynological and archeological data as a period of major environmental and cultural change, the cause of which is not known. If such a disruption of the climate system were to recur today, the social, economic, and political consequences would be nothing short of catastrophic (Bryson and Murray, 1977).

If climate is considered from a mathematical viewpoint, it is theoretically possible that a particular set of boundary conditions (solar radiation receipts, Earth surface conditions, etc.) may not give rise to a unique climatic state. Two or more distinct sets of statistics ("climate") may result from a single set of controls on the atmospheric circulation (Lorenz, 1968, 1970, 1976). In a practical sense, this suggests that climate (taken here to mean a particular mode of the general circulation of the atmosphere) may be essentially stable until some external factor (e.g., a change in solar radiation output or in volcanic dust loading of the atmosphere) causes a perturbation in the system. This perturbation may be only a short-term phenomenon, after which boundary conditions return to their former state; however, the resultant climate may not be the same, even though the boundary conditions are nearly identical to those before the perturbation — one of the other "solutions" to the mathematical problem of climate may have been adopted. Such a system is said to be intransitive. If, on the other hand, there is only one unique climatic state corresponding to a given set of boundary conditions, the system is said to be transitive. If climate does operate as an intransitive system, this poses intractable problems for mathematical models of climate, and for attempts to use these for climate forecasting. There is another possibility that complicates matters further. With only a minor change in boundary conditions, it is theoretically possible for there to be two or more time-dependent solutions, each with different statistics (climate) when considered over a moderate time span (i.e., the system may appear to be intransitive). However, if the time span is made sufficiently long, the different statistics converge to an essentially stable state. This is referred to as an almost-intransitive system (Lorenz, 1968). In practice, this means that a single set of boundary conditions may result in different climate states over discrete time intervals. If we were observing these different states through the paleoclimatic record, they would appear to represent periods separated by a "climatic change" (implying an external causative factor) whereas they would be, in actuality, merely stages on the way to a long-term stable state.

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